1. Field of the Invention
This invention relates to an intake air flow rate detecting device for an internal combustion engine, and more particularly to an intake air flow rate detecting device for detecting the flow rate of intake air using a Karman vortex flowmeter.
2. Description of the Prior Art
There is an increasing demand for an intake air flow rate detecting device which can detect the flow rate of intake air with a high accuracy in order to effect various controls in an internal combustion engine, e.g., control of the amount of fuel to be injected.
There has been proposed, in Japanese Unexamined Patent Publication No. 56(1981)-167824, an intake air flow rate detecting device utilizing a Karman vortex flowmeter. As is well known, the Karman vortex flowmeter generates an output having a frequency corresponding to a Karman vortex frequency f the value of which is given by the following equation wherein the air flow velocity, the width of a trail generator and the Strouhal number are respectively represented by V, d and Sr. EQU f=Sr.times.V/d
By detecting the Karman vortex frequency f, the air flow velocity V can be known and accordingly the flow rate Q of intake air (Q=AV, A representing the cross sectional area of the flow passage) can be known.
The said Strouhal number Sr varies with the Reynolds number Re and the relation therebetween is shown in FIG. 1. Further, the Reynolds number Re varies according to the air flow velocity V and the kinematic viscosity .nu. of intake air and when the internal diameter of the flowmeter is represented by D, the value of the Reynolds number Re is given by the following equation. EQU Re=V.times.D/.nu.
When the Reynolds number Re is held within the region A in FIG. 1, the Strouhal number Sr is kept substantially constant, and accordingly the flow rate Q of intake air can be determined with accuracy to some extent even if changes of the air flow velocity V and the kinematic viscosity .nu. should be neglected. However, in an engine installed on a vehicle, the flow rate Q of intake air varies over a very wide range from the minimum during idling to the maximum during full throttle travel, and the air flow velocity V also varies over a wide range. Accordingly, the value of the Reynolds number Re cannot be held within the region A of FIG. 1 and varies over a wide range out of the region A into the region B of FIG. 1 where the value of the Strouhal number Sr changes significantly according to the value of the Reynolds number Re.
As can be seen from the foregoing description, the flow rate Q of intake air can be known by way of the Karman vortex frequency f since the Karman vortex frequency f is in proportion to the air flow velocity V (and accordingly to the flow rate Q) with the Strouhal number Sr being the constant of proportionality. However the proportionality between the Karman vortex frequency f and the flow rate Q is broken, as shown in the region C in FIG. 2, when the flow rate Q is in such a range as to give a Reynolds number Re within the region B. In order to compensate for the fluctuation in the Strouhal number Sr due to the change of the air flow velocity V or the flow rate Q, conventionally, there has been effected a correction, when a Karman vortex frequency f corresponding to a flow rate Q in such a range as to fluctuate the Strouhal number Sr is obtained, to reduce the obtained Karman vortex frequency f or to reduce the width of a fuel injecting pulse as shown in FIG. 3 when the amount of fuel to be injected is controlled on the basis of the Karman vortex frequency f representing the flow rate Q. See Japanese Unexamined Patent Publication described above.
However, the Reynolds number Re is affected not only by the air flow velocity V but also by the kinematic viscosity .nu. of intake air as described above. That is, the kinematic viscosity .nu. also affects the Strouhal number Sr. Therefore, the flow rate Q of intake air cannot be measured with a high accuracy without taking into account the fluctuation in the Strouhal number Sr due to the change of the kinematic viscosity .nu. of intake air.